A closed level control system for a vehicle is disclosed in U.S. Pat. No. 6,685,174, and incorporated herein by reference. The level control system known from this application includes an air dryer via which air can be drawn from the atmosphere by suction with the aid of the compressor and can be transferred into the pressurized medium supply vessel for leakage compensation. Pressurized air can be discharged from the pressurized medium supply vessel into the atmosphere via the air dryer to regenerate the latter. With the integration of the air dryer into the level control system, it is substantially ensured that dehumidified air is in the level control system so that the components of the level control system are protected from corrosion and icing. It has, however, been determined that in a level control system known from the above publication, the air dryer directly faces the atmosphere so that the air dryer continuously takes up atmospheric moisture from the atmosphere. Under some circumstances, this can lead to the situation that the air dryer does not fulfill its function or cannot fulfill its function adequately. This problem could be solved in that the air dryer is correspondingly dimensioned but this would lead to a complicated configuration and high costs of the air dryer. 
It is an object of the invention to provide a closed level control system which includes an air dryer which is always available for air drying and furthermore has a simple configuration.
The closed level control system of the invention is for a vehicle having a vehicle body, vehicle axles and pressurized medium chambers with which the vehicle body is suspended relative to corresponding ones of the vehicle axles. The closed level control system includes: a pressurized medium supply vessel for holding a medium under pressure as a pressurized medium; a compressor for transferring the pressurized medium between the supply vessel and the pressurized medium chambers; the compressor having an input and an output; a first controllable directional valve switchable between a base position wherein no throughflow is permitted and a switched position wherein throughflow is permitted; a first pressurized air line connecting the supply vessel to the input of the compressor via the first controllable directional valve; a second controllable directional valve switchable between a base position wherein no throughflow is permitted and a switched position wherein throughflow is permitted; a second pressurized air line connecting the output of the compressor to the pressurized medium chambers via the second controllable directional valve; the pressurized medium being transmitted from the supply vessel to the pressurized medium chambers by the compressor when the first and second controllable directional valves are in the switched position; a third controllable directional valve switchable between a base position wherein no throughflow is permitted and a switched position wherein throughflow is permitted; a third pressurized air line connecting the input of the compressor to the pressurized medium chambers via the third controllable directional valve; a fourth controllable directional valve switchable between a base position wherein no throughflow is permitted and a switched position wherein throughflow is permitted; a fourth pressurized air line connecting the output of the compressor to the pressurized medium supply vessel via the fourth controllable directional valve; the pressurized medium being transmitted from each of the pressurized medium chambers to the supply vessel by the compressor when the third and fourth controllable directional valves are in the switched position; an air dryer mounted in the fourth pressurized air line; an intake valve switchable between a base position wherein no throughflow is permitted and a switched position wherein throughflow is permitted; an intake line ending at the intake valve and connecting the input of the compressor to the atmosphere when the intake valve is in the switched position; a discharge valve switchable between a base position wherein no throughflow is permitted and a switched position wherein throughflow is permitted; a discharge line branching off from the fourth pressurized air line at a branch point between the output of the compressor and the air dryer and ending at the discharge valve; and, the supply vessel being connectable to the atmosphere via the air dryer and the discharge line when the discharge valve is in the switched position.
An advantage of the invention is that the air dryer can be mounted in a pressurized air line of the level control system and is at least shielded completely from the atmosphere by the intake valve or by the discharge valve. In this way, the problem is avoided that the air dryer continuously takes up atmospheric moisture from the atmosphere. A further advantage of the invention is that the air dryer is mounted on the compressor output end (that is, on the pressure end of the compressor). This leads to an especially high effectiveness of the air dryer during drying operation. For this reason, the air dryer can have a simple configuration and a small-dimensioned dryer bed. A further advantage of the invention is that even though the air dryer is completely integrated into the closed level control system, pressurized air from the pressurized air supply vessel can be discharged via the air dryer (for its regeneration) into the atmosphere. A further advantage of the invention is that there is a flowthrough of the air dryer during regeneration operation in the opposite direction than during the drying operation. In this way, an excellent regeneration of the air dryer is ensured.
In the closed level control system of the invention, the first to fourth controllable directional valves are mounted in the first to fourth pressurized air lines, respectively; the first to fourth pressurized air lines are cleared in a first switching state of the first to fourth controllable directional valves and blocked in a second switching state of the first to fourth directional valves; the first and fourth pressurized air lines are connected together at a first point between the first and fourth controllable directional valves and the pressurized medium supply vessel and, from the first point, the first and fourth pressurized air lines are connected to the pressurized medium supply vessel via a first common pressurized air line; and, the second and third pressurized air lines are connected together at a second point between the second and third controllable directional valves and the pressurized medium chambers and, from the second point, the second and third pressurized air lines are connected to the pressurized medium chambers via a second common pressurized air line. The advantage of this embodiment is that only 2/2 directional valves are mounted in the four pressurized air lines which provide a simple configuration and are cost effective.
The closed level control system of the invention further comprises: the first and fourth controllable directional valves are combined to form a first composite directional valve; the second and third controllable directional valves are combined to form a second composite directional valve; each of the first and second composite controllable directional valves are switchable between first and second switching states; the first and second controllable composite directional valves can assume at least two switching states; the first pressurized air line is closed with the first controllable composite directional valve and the second pressurized air line is cleared with the second controllable composite directional valve; the fourth pressurized air line is blocked with the first controllable composite directional valve and the third pressurized air line is blocked with the second controllable composite directional valve when pressurized air is to be transferred from the pressurized medium chambers; and, the first and second controllable composite directional valves are a first switching state; the third pressurized air line is cleared with the second controllable composite directional valve and the fourth pressurized air line is cleared with the first controllable composite directional valve; and, the first pressurized air line is blocked with the first controllable composite directional valve and the second pressurized air line is blocked with the second controllable composite directional valve when pressurized air is to be transferred from the pressurized medium chambers to the pressurized medium supply vessel; and, the first and second controllable composite directional valves are in a second switching state. The advantage of this embodiment is that only two 3/2 directional valves are mounted in the four pressurized air lines which provide a simple configuration and are cost effective.
The closed level control system of the invention further includes: the first pressurized air line starting from the pressurized medium supply vessel and the third pressurized air line starting from the pressurized medium chambers are both terminated in a common third point connected directly to the input of the compressor.
In this embodiment, the first pressurized air line, which starts from the pressurized air vessel, and the third pressurized air line, which starts from the pressurized air chamber, end in a common point which is directly connected to the compressor input. The advantage of this embodiment is that, starting from the common point, the first pressurized air line and the third pressurized air line are led in a common pressurized air line to the compressor input and thereby the number of pressurized air lines needed is reduced.
The closed level control system of the invention further includes: a first check valve open to the input of the compressor; the first check valve is disposed in the first pressurized air line between the third point and the first controllable composite directional valve; a second check valve open to the input of the compressor; and, the second check valve is disposed in the third pressurized air line between the third point and the second controllable composite directional valve.
The advantage of this embodiment is that, in a specific position of the two 3/2 directional valves, an overflow of pressurized air from the pressurized air chambers into the pressurized air vessel is impossible and this is independent of whether the air pressure in the pressurized medium chambers is greater or less than the air pressure in the pressurized medium supply vessel. In the rest condition of the level control system, that is, when no control takes place, the controllable directional valves can, for example, be transferred into this position so that an unintended overflow of pressurized air between the pressurized medium chambers and the pressurized medium supply vessel cannot take place. Furthermore, with a defined state of this kind of the controllable directional valves, a pressure measurement of the pressure in the pressurized medium chambers is possible in a simple manner with the aid of a pressure sensor (in this connection, reference can be made to FIG. 2). The advantages achieved with this embodiment are attained while maintaining all functions of the level control system. It is furthermore especially possible to discharge pressurized air into the atmosphere from the pressurized medium supply vessel via the air dryer in order to regenerate the air dryer. For more information, reference can be made to the description of FIG. 2.
According to another feature of the invention, the level control system includes a further controllable directional valve which blocks the connection between the air springs and the compressor and air dryer but permits a connection between the air springs and the pressure sensor. An exchange of air and therefore a pressure compensation between the air dryer and/or compressor and the air springs (for example, for a pressure measurement in an air spring) can, when the pressure level of the air dryer or compressor and the particular air spring is different, lead to a pressure change in the particular air spring because of the volume of the air dryer or compressor. This generates a tilted position of the vehicle and therefore makes a readjustment of the vehicle level necessary. The advantage of this embodiment is that no air can escape into the air dryer or the compressor when there is a pressure measurement in an air spring (independent of the switching state of the two 3/2 directional valves) and therefore no pressure change can take place in the particular air spring because the additional controllable directional valve blocks this connection and therefore no inclined position of the vehicle is generated and no readjustment of the vehicle level is necessary after a pressure measurement.
According to another feature of the invention, the discharge line is connected to the intake line. A directional valve is mounted between the point at which the discharge line branches off from the fourth pressurized air line and the point at which the pressurized air line is connected to the intake line. This directional valve blocks the discharge line in a first switching state and clears the discharge line in a second switching state so that the intake valve can be simultaneously used as a discharge valve. The advantage of this further improvement of the first embodiment is that no separate intake valve need be made available.
According to another feature of the invention, the intake valve is disposed in a pressurized air line which branches from the second common pressurized air line. The advantage of this improvement is that the intake valve lies in direct proximity to those valves which are connected ahead of the individual pressurized medium chambers.
According to another embodiment of the invention, the discharge line between the point at which it branches from the fourth pressurized air line and the point at which it is connected to the intake line matches with the second pressurized air line. The advantage of this improvement is that the second pressurized air line can be used twice (once as a discharge line and once as a connecting piece of the pressurized medium supply vessel with the pressurized medium chambers) so that the length of the pressurized air lines in the level control system can be reduced.
The closed level control system of the invention further includes: the first and fourth controllable directional valves are combined to form a first composite directional valve; the second and third controllable directional valves are combined to form a second composite directional valve; each of the first and second composite controllable directional valves are switchable between first and second switching states; the first pressurized air line is cleared with the first composite directional valve and the second pressurized air line is cleared with the second composite directional valve when pressurized air is to be transferred from the pressurized medium supply vessel into the pressurized medium chambers with both of the composite directional valves being in the first switching states; and, the third pressurized air line is cleared with the first composite directional valve and the fourth pressurized air line is cleared with the second composite directional valve when pressurized air is to be transferred from the pressurized medium chambers into the pressurized medium supply vessel with both of the composite directional valves being in the second switching state.
According to another feature of the invention, a check valve, which is opening toward the air dryer, is mounted in the fourth pressurized air line between the air dryer and the pressurized medium supply vessel. The check valve is bridged by a pressurized air line in which a throttle is mounted. The advantage of this improvement is that the pressurized air is guided through the throttle during the transfer of pressurized air from the pressurized medium supply vessel via the air dryer into the atmosphere, so that the pressurized air is relaxed before entry into the air dryer. In this way, an especially good regeneration of the air dryer is ensured and only a small air quantity need be discharged in order to completely regenerate the air dryer. Nevertheless, it is ensured that pressurized air can be transferred rapidly from the pressurized medium chambers into the pressurized medium supply vessel so that a rapid drop of the vehicle body is possible. This is so because, in this case, the pressurized air is transferred starting from the air dryer via the check valve into the pressurized medium supply vessel.
According to another improvement of the invention, the level control system includes a pressure sensor which is mounted in a pressurized air line starting from the pressurized medium chambers. A controllable directional valve is disposed between each pressurized medium chamber and the pressure sensor. In a first switching state, the directional valve separates the pressurized medium chambers from the pressure sensor and, in a second switching state, the pressurized medium chamber is connected to the pressure sensor. The advantage of this improvement is that the pressure in the pressurized medium chambers can be measured in a simple manner with the pressure sensor. Furthermore, it is likewise possible to measure the pressure in the pressurized medium supply vessel with the aid of the pressure sensor.
In the level control systems of the type initially mentioned herein, it is further the requirement that the control operations be carried out as rapidly as possible. This concerns not only control operations during travel but also those at standstill of the vehicle or where travel is at a low speed. Often, great differences in elevation must be mastered. Accordingly, for a relatively high built vehicle (such as the so-called sports utility vehicles), the desire is present to lower the vehicle when stopping in order to facilitate a comfortable exiting and entering of the driver and passenger and to raise the vehicle to an optimal travel height again when again starting travel. In conventional level control systems, it takes a relatively long time until the air is withdrawn from all pressurized medium chambers so that the passengers are compelled to wait after stopping the vehicle before they can comfortably exit therefrom. Closed level control systems are here already at an advantage compared to open systems wherein the air can flow off only slowly via the air dryer.
According to a further feature of the invention, the level control system is equipped with several compressors. Each compressor is itself connected to the pressurized medium supply vessel in the manner described above and to specific pressurized medium chambers. In this way, a single compressor is no longer responsible for the entire system with all pressurized medium chambers; instead, an individual compressor only has to supply a portion of the pressurized medium chambers and, in the extreme case, even only a single pressurized medium chamber. Several compressors can simultaneously supply air to the pressurized medium chambers or air can be withdrawn from the pressurized medium chambers. In a parallel operation of this kind, these operations can take place much faster than in conventional level control systems wherein all the air is guided via a single compressor. The improvement of the invention in accordance with this embodiment therefore has the advantage that one can introduce a level change of the vehicle more rapidly than with a conventional level control system. This improvement of the invention has the further advantage that air is supplied to a portion of the pressurized medium chambers and, at the same time, air can be withdrawn from other pressurized medium chambers and air can be exchanged between individual pressurized medium chambers or groups of pressurized medium chambers. Compared to the system of the first embodiments (which make possible an individual driving of the individual air springs but only either in the operation of xe2x80x9cfillingxe2x80x9d or in the operation of xe2x80x9cdischargexe2x80x9d, but not in both modes of operation simultaneously), there results in this way the advantage of expanded control possibilities. With this system, excellent possibilities are provided to carry out further programs for controlling (open loop and/or closed loop) the body of the vehicle which go beyond the control of the level.
According to another embodiment, a compressor is allocated to the pressurized medium chambers of each axle. This embodiment has the advantage that rapid pressure changes can be achieved with little complexity. For a vehicle having two axles, a doubling of the speed for level changes compared to conventional systems are achieved with only one compressor. According to a further embodiment, a compressor is assigned even to the pressurized medium chambers of each wheel of the vehicle. This embodiment has the advantage that still more rapid reactions are possible and that, in addition, individual controls for each wheel can be undertaken completely independently of the other pressurized medium chambers.
According to another improvement of the invention, only one air dryer as well as one intake line and one discharge line with corresponding valves are provided for the entire level control system. This embodiment has the advantage that the structural complexity is held low. This is based on the recognition that an air exchange with the ambient takes place to only a small extent for a closed level control system and that all components of the system are connected to each other via the pressurized medium supply vessel.
According to another embodiment of the invention, the pressurized medium chambers of the individual vehicle axles are assigned their own intermediate stores for the pressurized medium. When dropping the vehicle, these intermediate stores can take up a large quantity of air from the pressurized medium chambers in a short time whereby a rapid lowering of the vehicle is possible. From the intermediate stores, the air can be moved via the compressor into the pressurized medium supply vessel. This transport of the air can take place slowly without the lowering of the vehicle being slowed. In the system, the discharge of the pressurized medium chambers for lowering the level of the vehicle is decoupled from the flow of the pressurized medium into the remaining parts of the system (to which the compressor, the valves and relatively long line systems belong). A rapid discharge of the pressurized medium chambers is possible without it being necessary to design the entire system for a rapid displacement of large quantities of air. Compared to the alternative embodiments, this embodiment has the advantage that only a single compressor is required.
Preferably, the output lines of the intermediate stores are provided with check valves which permit an unimpeded discharge of the pressurized medium from the intermediate stores but prevent an inflow of the air from the system. In this way, it is achieved that the pressure in the intermediate stores is always less than in the pressurized medium chambers.
According to a further embodiment of the invention, the individual pressurized medium chambers are assigned respective directional valves. These valves are connected to respective pressurized medium lines leading to the input of the compressor. With these valves, a control to compensate for pitch and roll movements of the vehicle is made possible as explained hereinafter.
According to another improvement of the invention, a line is provided which serves to connect external apparatus. This line is connected to the pressurized medium lines. This further embodiment of the invention affords the advantage that the pressurized air in the system can be utilized for additional apparatus. In an advantageous manner, the line for connecting to external apparatus is equipped with a lock. The lock is only opened when an external apparatus is connected to the line. Preferably, the line leading to the outside functions for connecting a tire filling device.
According to a further feature, the line, which serves for connecting, is connected directly to the compressor output. This embodiment affords the advantage that air, which is drawn by suction from the ambient when pumped to the external connection by the compressor, does not flow through the air dryer and the air dryer is therefore not subjected to unnecessary loading.